The present invention relates to the control of a device which operates upon articles moved past the device. In particular, the invention relates to the control of heating apparatus of the type used for drying articles, such as discrete sheet material. The invention has particular application to the infrared drying of printed sheets in a printing press.
Specifically, the invention relates to the control of dryers for printing presses comprising infrared heating units arranged to face the printed sheets as they are conveyed past the dryer. In one common control arrangement, the heating unit is turned on to a preselected intensity in response to the operation of the impression rolls of the press. In this arrangement, the heater simply stays on, as long as the impression rolls are operative. Therefore, in the case of a paper feed malfunction, for example, the heater would stay on despite the fact that no paper is being fed through the press, resulting in a waste of power for operating the heaters. Similarly, the dryer does not turn off in the event of a paper jam. This can be dangerous since, if the jam occurs in the vicinity of the heater the intense heat created thereby can easily start a fire.
It has, therefore, been recognized that it is desirable to tie the control of the dryer to the movement of the paper rather than to the operation of the impression rolls of the press. One such arrangement is disclosed in U.S. Pat. Nos. 4,354,095 and 4,435,637. The dryer control circuits in these patents utilize two types of controls. The intensity of the heaters is varied in response to either the speed of the paper conveyor or the temperature of the paper at the detecting zone. This temperature is also a function of the transport speed, since the slower the speed the longer a sheet remains in front of the heater and the higher the temperature to which it will be raised. The control circuits also utilize a capacitive discharge timing circuit to disconnect the heater if one or more sheets is missing from a series of conveyed sheets, indicating a misfeed, or if a sheet remains too long in front of the sensor, indicating a jam.
But these prior control circuits are sensitive to the gripper bars of the paper carriers, as well as to the paper sheets themselves. This makes it very difficult to properly set the RC time constant for detecting the absence of a sheet. This time constant must be greater than the time intervals for passage of the gap between the end of a sheet and the next carrier, and yet be less than the time interval for passage of the gap between adjacent carriers. Since both of these time intervals vary with the speed of the conveyor, a proper setting for the time constant is difficult to determine and maintain in use.
An attempt has been made to avoid this difficulty by sensing the free tail end of a gripped sheet, this tail end being distinguishable because it droops from the plane of the leading edge of the sheet. But the amount of droop also varies with the speed of the press conveyor.